This invention relates generally to xerographic copying apparatus and more particularly, it relates to the heat and pressure fixing of particulate thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto.
In order to affix or fuse electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent into the fibers or pores of the support members or otherwise on the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated.
Heat fixing devices of electronic copying machines are generally of a construction, as shown in FIG. 1, such that a paper b with transferred toner images is urged by a pressure roll c against a fixing roll a which contains a heat source d. By the application of pressure and heat of the fixing roll a the toner image is fused and fixed on the paper b. To this end, the fixing roll a is required to have a high strength, for example, it will have a wall thickness large enough to withstand a high pressure. However, where the circumferential wall thickness is relatively large so as to be able to withstand the pressures to which it is subjected, it has a correspondingly large heat capacity resulting in a prolonged time being required between the time when heating is started until when the fusing temperature is reached, thus making immediate copying impossible.